Torque vectoring drive axle assembly

ABSTRACT

A drive axle assembly includes first and second axleshafts connected to a pair of wheels and a drive mechanism operable to selectively couple a driven input shaft to one or both of the axleshafts. The drive mechanism includes a differential, a speed changing unit operably disposed between the differential assembly and one of the first and second axleshafts, and first and second mode clutches. The first mode clutch is operable to decrease the rotary speed of the second axleshaft which, in turn, causes a corresponding increase in the rotary speed of the first axleshaft. The second mode clutch is operable to increase the rotary speed of the second axleshaft so as to cause a decrease in the rotary speed of the first axleshaft. A control system controls actuation of both mode clutches.

FIELD OF THE INVENTION

The present invention relates generally to differential assemblies foruse in motor vehicles and, more specifically, to a differential assemblyequipped with a torque vectoring drive mechanism and an active controlsystem.

BACKGROUND OF THE INVENTION

In view of consumer demand for four-wheel drive vehicles, many differentpower transfer system are currently utilized for directing motive power(“drive torque”) to all four-wheels of the vehicle. A number of currentgeneration four-wheel drive vehicles may be characterized as includingan “adaptive” power transfer system that is operable for automaticallydirecting power to the secondary driveline, without any input from thevehicle operator, when traction is lost at the primary driveline.Typically, such adaptive torque control results from variable engagementof an electrically or hydraulically operated transfer clutch based onthe operating conditions and specific vehicle dynamics detected bysensors associated with an electronic traction control system. Inconventional rear-wheel drive (RWD) vehicles, the transfer clutch istypically installed in a transfer case for automatically transferringdrive torque to the front driveline in response to slip in the reardriveline. Similarly, the transfer clutch can be installed in a powertransfer device, such as a power take-off unit (PTU) or in-line torquecoupling, when used in a front-wheel drive (FWD) vehicle fortransferring drive torque to the rear driveline in response to slip inthe front driveline. Such adaptively-controlled power transfer systemcan also be arranged to limit slip and bias the torque distributionbetween the front and rear drivelines by controlling variable engagementof a transfer clutch that is operably associated with a centerdifferential installed in the transfer case or PTU.

To further enhance the traction and stability characteristics offour-wheel drive vehicles, it is also known to equip such vehicles withbrake-based electronic stability control systems and/or tractiondistributing axle assemblies. Typically, such axle assemblies include adrive mechanism that is operable for adaptively regulating theside-to-side (i.e., left-right) torque and speed characteristics betweena pair of drive wheels. In some instances, a pair of modulatableclutches are used to provide this side-to-side control, as is disclosedin U.S. Pat. Nos. 6,378,677 and 5,699,888. According to an alternativedrive axle arrangement, U.S. Pat. No. 6,520,880 discloses ahydraulically-operated traction distribution assembly. In addition,alternative traction distributing drive axle assemblies are disclosed inU.S. Pat. Nos. 5,370,588 and 6,213,241.

As part of the ever increasing sophistication of adaptive power transfersystems, greater attention is currently being given to the yaw controland stability enhancement features that can be provided by such tractiondistributing drive axles. Accordingly, this invention is intended toaddress the need to provide design alternatives which improve upon thecurrent technology.

SUMMARY OF THE INVENTION

Accordingly, it is an objective of the present invention to provide adrive axle assembly for use in motor vehicles which is equipped with anadaptive yaw control system.

To achieve this objective, the drive axle assembly of the presentinvention includes first and second axleshafts connected to a pair ofwheels and a torque distributing drive mechanism that is operable fortransferring drive torque from a driven input shaft to the first andsecond axleshafts. The torque distributing drive mechanism includes adifferential, a speed changing unit, and first and second mode clutches.The differential includes an input component driven by the input shaft,a first output component driving the first axleshaft and a second outputcomponent driving the second axleshaft. The speed changing unit includesa first shaft commonly driven with the first axleshaft, a second shaftcommonly driven with the second axleshaft, and first and second gearsetsdriven by the first shaft. The first mode clutch is operable forselectively coupling the first gearset to the second shaft. Likewise,the second mode clutch is operable for selectively coupling the secondgearset to the second shaft. Accordingly, selective control overactuation of one or both of the first and second mode clutches providesadaptive control of the speed differentiation and the torque transferredbetween the first and second axleshafts. A control system including andECU and sensors are provided to control actuation of both mode clutches.

According to one preferred embodiment, the first gearset of the speedchanging unit is an underdrive unit that is operable to decrease therotary speed of the second shaft relative to the first shaft. Likewise,the second gearset of the speed changing unit is an overdrive unit thatis operable to increase the rotary speed of the second shaft relative tothe first shaft. As such, engagement of the first mode clutch results inthe second axleshaft being underdriven relative to the first axleshaft.In contrast, engagement of the second mode clutch results in the secondaxleshaft being overdriven relative to the first axleshaft.

Pursuant to an alternative objective of the present invention, thetorque distributing drive mechanism can be utilized in a power transferunit, such as a transfer case, of a four-wheel drive vehicle toadaptively control the front-rear distribution of drive torque deliveredfrom the powertrain to the front and rear wheels.

Further objectives and advantages of the present invention will becomeapparent by reference to the following detailed description of thepreferred embodiments and the appended claims when taken in conjunctionwith the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from thedetailed description and the accompanying drawings, wherein:

FIG. 1 is a diagrammatical illustration of an all-wheel drive motorvehicle equipped with a drive axle having a torque distributingdifferential assembly and an active yaw control system according to thepresent invention;

FIG. 2 is a schematic illustration of the torque distributingdifferential assembly shown in FIG. 1;

FIG. 3 is another illustration of the torque distributing differentialassembly shown in FIGS. 1 and 2;

FIG. 4 is a diagrammatical illustration of the power-operated actuatorsassociated with the torque distributing differential assembly of thepresent invention;

FIG. 5 is a schematic illustration of an alternative embodiment of thetorque distributing differential assembly of the present invention;

FIG. 6 is a diagrammatical illustration of the torque distributingdifferential assembly of the present invention installed in a powertransfer unit for use in a four-wheel drive vehicle; and

FIG. 7 is a schematic drawing of the power transfer unit shown in FIG.6.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, an all-wheel drive vehicle 10 includes an engine 12transversely mounted in a front portion of a vehicle body, atransmission 14 provided integrally with engine 12, a front differential16 which connects transmission 14 to front axleshafts 18L and 18R andleft and right front wheels 20L and 20R, a power transfer unit (“PTU”)22 which connects front differential 16 to a propshaft 24, and a rearaxle assembly 26 having a torque distributing drive mechanism 28 whichconnects propshaft 24 to axleshafts 30L and 30R for driving left andright rear wheels 32L and 32R. As will be detailed, drive mechanism 28is operable in association with a yaw control system 34 for controllingthe transmission of drive torque through axleshafts 30L and 30R to rearwheels 32L and 32R.

In addition to an electronic control unit (ECU) 36, yaw control system34 includes a plurality of sensors for detecting various operational anddynamic characteristics of vehicle 10. For example, a front wheel speedsensor 38 is provided for detecting a front wheel speed value based onrotation of propshaft 24, a pair of rear wheel speed sensors 40 areoperable to detect the individual rear wheel speed values based rotationof left and right axle shafts 30L and 30R, and a steering angle sensor42 is provided to detect the steering angle of a steering wheel 44. Thesensors also include a yaw rate sensor 46 for detecting a yaw rate ofthe body portion of vehicle 10, a lateral acceleration sensor 48 fordetecting a lateral acceleration of the vehicle body, and a lock switch50 for permitting the vehicle operator to intentionally shift drivemechanism 28 into a locked mode. As will be detailed, ECU 36 controlsoperation of a pair of mode clutches associated with drive mechanism 28by utilizing a control strategy that is based on input signals from thevarious sensors and lock switch 50.

Rear axle assembly 26 includes an axle housing 52 within which drivemechanism 28 is rotatably supported. In general, torque distributingdrive mechanism 28 includes an input shaft 54, a differential 56, aspeed changing unit 58, a first mode clutch 60 and a second mode clutch62. As seen, input shaft 54 includes a pinion gear 64 that is inconstant mesh with a hypoid ring gear 66. Ring gear 66 is fixed forrotation with a drive case 68 associated with differential 56. As seen,differential 56 is a planetary gearset having an annulus ring gear 70fixed for common rotation with drive case 68, a sun gear 72 fixed forrotation with right axleshaft 30R, a differential carrier 74 fixed forrotation with left axleshaft 30L, and meshed pairs of first planet gears76 and second planet gears 78. First planet gears 76 are shown to bemeshed with sun gear 72 while second planet gears 78 are meshed withannulus ring gear 70. Differential carrier 74 is a multi-piece assemblyhaving a front carrier ring 80 interconnected to a rear carrier ring 82with first and second pins 84 and 86, respectively, extendingtherebetween and on which corresponding first and second planet gears 76and 78 are rotatably supported. Differential 56 is operable to transferdrive torque from drive case 68 to axleshafts 30L and 30R at a ratiodefined by the gear components while permitting speed differentiationtherebetween. Preferably, a 50/50 torque split ratio is established bydifferential 56 for use in this particular drive axle application. Thus,differential 56 has ring gear 70 as its input component, differentialcarrier 74 as its first output component and sun gear 72 as its secondoutput component. However, it should be understood that differential 56is merely intended to represent one differential arrangement applicablefor use with the present invention and that other know planetary andhypoid-type differentials could be substituted for use with the presentinvention.

Speed changing unit 58 includes a first transfer shaft 90 driven bydifferential carrier 74 for common rotation with left axleshaft 30L, asecond transfer shaft 92 operably connected for rotation with rightaxleshaft 30L via a transfer unit 94, a first constant-mesh gearset 96and a second constant-mesh gearset 98. Transfer unit 94 includes a firsttransfer gear 100 coupled for rotation with second transfer shaft 92, asecond transfer gear 102 coupled for rotation with right axleshaft 30R,and an idler gear 104 meshed with both of first transfer gear 100 andsecond transfer gear 102. First gearset 96 includes a first drive gear106 that is fixed to first transfer shaft 90 and meshed with a firstspeed gear 108 that is rotatably supported on second transfer shaft 92.In essence, first gearset 96 is a speed reducing or “underdrive” gearsetwhich functions to cause first speed gear 108 to be rotatably driven ata slower rotary speed than first transfer shaft 90. Likewise, secondgearset 98 includes a second drive gear 110 that is fixed to firsttransfer shaft 90 and meshed with a second speed gear 112 that isrotatably supported on second transfer shaft 92. In contrast to firstgearset 96, second gearset 98 is a speed increasing or “overdrive”gearset which functions to cause second speed gear 112 to be driven at afaster rotary speed than first transfer shaft 90.

With continued reference to FIGS. 2 and 3, first mode clutch 60 is shownto be operably disposed between first speed gear 108 of first gearset 96and second transfer shaft 92. In particular, first mode clutch 60includes a clutch hub 114 that is connected to first speed gear 108 anda drum 116 that is fixed for rotation with second transfer shaft 92.First mode clutch 60 also includes a multi-plate clutch pack 118 that isoperably disposed between drum 116 and hub 114, and a power-operatedclutch actuator 120. First mode clutch 60 is operable in a first or“released” mode so as to permit unrestricted rotation of second transfershaft 92 relative to first transfer shaft 90. In contrast, first modeclutch 60 is also operable in a second or “locked” mode to couple firstspeed gear 108 to second transfer shaft 92, thereby driving secondtransfer shaft 92 at a reduced speed relative to first transfer shaft90. Thus, first mode clutch 60 functions in its locked mode to decreasethe rotary speed of right axleshaft 30R which, in turn, causesdifferential 56 to generate a corresponding increase in the rotary speedof left axleshaft 30L, thereby directing more drive torque to leftaxleshaft 30L than is transmitted to right axleshaft 30R. First modeclutch 60 is shifted between its released and locked modes via actuationof power-operated clutch actuator 120 in response to control signalsfrom ECU 36. Specifically, first mode clutch 60 is operable in itsreleased mode when clutch actuator 120 applies a predetermined minimumcutch engagement force on clutch pack 118 and is further operable in itslocked mode when clutch actuator 120 applies a predetermined maximumclutch engagement force on clutch pack 118.

Second mode clutch 62 is shown to be operably disposed between secondspeed gear 112 of second gearset 98 and second transfer shaft 92. Inparticular, second mode clutch 62 includes a clutch hub 122 that isfixed for rotation with second speed gear 112, a drum 124 fixed forrotation with second transfer shaft 92, a multi-plate clutch pack 126operably disposed between hub 122 and drum 124, and a power-operatedclutch actuator 128. Second mode clutch 62 is operable in a first or“released” mode so as to permit unrestricted relative rotation betweenfirst transfer shaft 90 and second transfer shaft 92. In contrast,second mode clutch 62 is also operable in a second or “locked” mode tocouple second speed gear 112 to second transfer shaft 92, therebyincreasing the rotary speed of second transfer shaft 92 relative tofirst transfer shaft 90. Thus, second mode clutch 62 functions in itslocked mode to increase the rotary speed of right axleshaft 30R which,in turn, causes differential 56 to decrease the rotary speed of leftaxleshaft 30L, thereby directing more drive torque to right axleshaft30R than is directed to left axleshaft 30L. Second mode clutch 62 isshifted between its released and locked modes via actuation ofpower-operated clutch actuator 128 in response to control signals fromECU 36. In particular, second mode clutch 62 operates in its releasedmode when clutch actuator 128 applies a predetermined minimum clutchengagement force on clutch pack 126 while it operates in its locked modewhen clutch actuator 128 applies a predetermined maximum clutchengagement force on cutch pack 126.

As seen, power-operated clutch actuators 120 and 128 are shown inschematic fashion to cumulatively represent the components required toaccept a control signal from ECU 36 and generate a clutch engagementforce to be applied to corresponding clutch packs 118 and 126. To thisend, FIG. 4 diagrammatically illustrates the basic components associatedwith such power-operated clutch actuators. Specifically, eachpower-operated actuator includes a controlled device 132, a forcegenerating mechanism 134, and a force apply mechanism 136. Inelectro-mechanical systems, controlled device 132 would represent suchcomponents as, for example, an electric motor or an electromagneticsolenoid assembly capable of receiving an electric control signal fromECU 36. The output of controlled device 132 would drive force generatingmechanism 134 which could include, for example, a ball ramp, a ballscrew, a leadscrew, a pivotal lever arm, rotatable cam plates, etc.,each of which is capable of converting the output of controlled device132 into a clutch engagement force. Finally, force apply mechanism 136functions to transmit and exert the clutch engagement force generated byforce generating mechanism 134 onto clutch packs 118 and 126 and caninclude, for example, an apply plate or a thrust plate. If ahydra-mechanical system is used, controlled device 132 could be anelectrically-operated control valve that is operable for controlling thedelivery of pressurized fluid from a fluid source to a piston chamber. Apiston disposed for movement in the piston chamber would act as forcegenerating mechanism 134. Preferably, controlled device 132 is capableof receiving variable electric control signals from ECU 36 forpermitting variable regulation of the magnitude of the clutch engagementforce generated and applied to the clutch packs so as to permit“adaptive” control of the mode clutches.

In accordance with the arrangement shown, torque distributing drivemechanism 28 is operable in coordination with yaw control system 34 toestablish at a least four distinct operational modes for controlling thetransfer of drive torque from input shaft 54 to axleshafts 30L and 30R.In particular, a first operational mode is established when first modeclutch 60 and second mode clutch 62 are both in their released mode suchthat differential 56 acts as an “open” differential so as to permitunrestricted speed differentiation with drive torque transmitted fromdrive case 68 to each axleshaft 30L, 30R based on the tractiveconditions at each corresponding rear wheel 32L and 32R. A secondoperational mode is established when both first mode clutch 60 andsecond mode clutch 62 are in their locked mode such that differential 56acts as a “locked” differential with no speed differentiation permittedbetween rear axleshafts 30L and 30R. This mode can be intentionallyselected via actuation of lock switch 50 when vehicle 10 is beingoperated off-road or on poor roads.

A third operational mode is established when first mode clutch 60 is inits locked mode while second mode clutch 62 is in its released mode. Asa result, right axleshaft 30R is underdriven due to the coupledengagement of first speed gear 108 to second transfer shaft 92. Asnoted, such a reduction in the rotary speed of right axleshaft 30Rcauses a corresponding speed increase in left axleshaft 30L. Thus, thisthird operational mode causes right axleshaft 30R to be underdrivenwhile left axieshaft 30L is overdriven when required to accommodate thecurrent tractive or steering condition detected and/or anticipated byECU 36 based on the particular control strategy used. Likewise, a fourthoperational mode is established when first mode clutch 60 is shiftedinto its released mode and second mode clutch 62 is shifted into itslocked mode. As a result, right rear axleshaft 30R is overdrivenrelative to drive case 68 which, in turn, causes left axleshaft 30L tobe underdriven at a corresponding reduced speed. Thus, this fourthoperational mode causes right axleshaft 30R to be overdriven while leftaxleshaft 30L is underdriven when required to accommodate the currenttractive or steering conditions detected and/or anticipated by ECU 36.

At the start of vehicle 10, power from engine 12 is transmitted to frontwheels 20L and 20R through transmission 14 and front differential 16.Drive torque is also transmitted to torque distributing drive mechanism28 through PTU 22 and propshaft 24 which, in turn, rotatably drivesinput pinion shaft 58. Typically, mode clutches 60 and 62 would benon-engaged such that drive torque is transmitted through differential56 to rear wheels 32L and 32R. However, upon detection of lost tractionat front wheels 20L and 20R, one or both mode clutches 60 and 62 can beengaged to provide drive torque to rear wheels 32L and 32R based on thetractive needs of the vehicles.

In addition to on-off control of the mode clutches to establish thevarious drive modes associated with overdrive connections through speedchanging unit 58, it is further contemplated and preferred that variableclutch engagement forces can be generated by power-operated actuators120 and 128 to adaptively regulate the left-to-right speed and torquecharacteristics. This “adaptive” control feature functions to provideenhanced yaw and stability control for vehicle 10. For example, areference yaw rate can be determined based on several factors includingthe steering angle detected by steering angle sensor 42, the vehiclespeed as calculated based on signals from the various speed sensors, anda lateral acceleration as detected by lateral acceleration sensor 48.ECU 36 compares this reference yaw rate with an actual yaw rate valuedetected by yaw sensor 46. This comparison will determine whethervehicle 10 is in an understeer or an oversteer condition, and theseverity of the condition, so as to permit yaw control system 34 to beadaptively control actuation of the mode clutches to accommodate thesesteering tendencies. ECU 36 can address such conditions by initiallyshifting drive mechanism 28 into one of the specific operational drivemode that is best suited to correct the actual or anticipated oversteeror understeer situation. Thereafter, variable control of the modeclutches permits adaptive regulation of the side-to-side torque transferand speed differentiation characteristics when one of the distinct drivemodes is not adequate to accommodate the current steer tractivecondition.

Referring now to FIG. 5, an alternative embodiment of torquedistributing drive mechanism 28 of FIG. 2 is shown and designated byreference numeral 28′. Generally speaking, a large number of componentsare common to both drive mechanism 28 and 28′, with such componentsbeing identified by the same reference numbers. However, a beveldifferential 56′ replaces planetary differential 56 and first transfershaft 90 is now shown to be driven by the input component of beveldifferential 56′ instead of one of the output components of planetarydifferential 56. Bevel differential 56′ includes a differential case 68′as its input component and a pair of left and right side gears 130L and130R, respectively, as its output components. Pinion gears 132 aredriven by differential case 68′ and mesh with both of side gears 130Land 130R.

Drive mechanism 28′ is also operable to establish the four operationaldrive modes previously disclosed. Specifically, with both first andsecond mode clutches 60 and 62 released, differential 56′ acts as anopen differential unit for transferring drive torque from differentialcase 68′ to axleshafts 30L and 30R based on the tractive conditions ateach wheel. Likewise, with both first and second mode clutches 60 and 62locked, differential 56′ is locked. The third drive mode is againestablished when first mode clutch 60 is engaged and second mode clutch62 is released such that the rotary speed of right axleshaft 30R isreduced relative to the rotary speed of first transfer shaft 90.Accordingly, bevel differential 56′ causes left axleshaft 30L to bedriven at a corresponding increased rotary speed. As such, rightaxleshaft 30R is underdriven and left axleshaft 30L is overdriven whenthe third drive mode is established. Finally, the fourth operationaldrive mode is established when first mode clutch 60 is released andsecond mode clutch 62 is locked such that the rotary speed of rightaxleshaft 30R is increased relative to that of first transfer shaft 90.Accordingly, bevel differential 56′ causes left axleshaft 30L to bedriven at a corresponding reduced rotary speed. Thus, right axleshaft30R is overdriven and left axleshaft 30L is underdriven when the fourthdrive mode is established.

Referring now to FIG. 6, a four-wheel drive vehicle 10′ is shownequipped with a power transfer unit 160 that is operable fortransferring drive torque from the output of transmission 14 to a first(i.e., front) output shaft 162 and a second (i.e., rear) output shaft164. Front output shaft 162 drives a front propshaft 166 which, in turn,drives front differential 16 for driving front wheels 20L and 20R.Likewise, rear output shaft 164 drives a rear propshaft 168 which, inturn, drives a rear differential 170 for driving rear wheels 32L and32R. Power transfer unit 160, otherwise known as a transfer case,includes a torque distributing drive mechanism 172 which functions totransmit drive torque from its input shaft 174 to both of output shafts162 and 164 so as to bias the torque distribution ratio therebetween,thereby controlling the tractive operation of vehicle 10′. As seen,torque distribution mechanism 172 is operably associated with a tractioncontrol system 34′ for providing this adaptive traction control featurefor vehicle 10′.

Referring primarily to FIG. 7, torque distribution mechanism 172 ofpower transfer unit 160 is shown to be generally similar in structure todrive mechanism 28 of FIG. 2 with the exception that drive case 68 isnow drivingly connected to input shaft 174 via a transfer assembly 180.In the arrangement shown, transfer assembly 180 includes a firstsprocket 182 driven by input shaft 174, a second sprocket 184 drivingdrive case 68, and a power chain 186 therebetween. As seen, front outputshaft 162 is driven by differential carrier 74 of differential 56 whichnow acts as a center or “interaxle” differential for permitting speeddifferentiation between the front and rear output shafts whileestablishing a full-time four-wheel drive mode. In addition, sun gear 72of differential 56 drives rear output shaft 164.

Control over actuation of mode clutches 60 and 62 results incorresponding increases or decreases in the rotary speed of rear outputshaft 164 relative to front output shaft 162, thereby controlling theamount of drive torque transmitted therebetween. In particular, whenboth mode clutches are released, unrestricted speed differentiation ispermitted between the front and rear output shafts while the gear ratioestablished by the components of interaxle differential 56 controls thefront-to-rear torque ratio based on the current tractive conditions ofthe front and rear wheels. In contrast, with both mode clutches engaged,a locked four-wheel drive mode is established wherein no interaxle speeddifferentiation is permitted between the front and rear output shafts.Such a drive mode can be intentionally selected via lock switch 50 whenvehicle 10′ is driven off-road or during severe road conditions. Anadaptive full-time four-wheel drive mode is made available under controlof traction control system 34′ to limit interaxle slip and vary thefront-rear drive torque distribution ratio based on the tractive needsof the front and rear wheels as detected by the various sensors. Inaddition to power transfer unit 160, vehicle 10′ could also be equippedwith a rear axle assembly having either torque distributing drivemechanism 28 or 28′ and its corresponding yaw control system, as isidentified by the phantom lines in FIG. 6.

The description of the invention is merely exemplary in nature and,thus, variations that do not depart from the gist of the invention areintended to be within the scope of the invention. Such variations arenot to be regarded as a departure from the spirit and scope of theinvention.

1. A motor vehicle, comprising: a powertrain operable for generatingdrive torque; a primary driveline for transmitting drive torque fromsaid powertrain to first and second primary wheels; a secondarydriveline for selectively transmitting drive torque from said powertrainto first and second secondary wheels, said secondary driveline includingan input shaft driven by said powertrain, a first axleshaft driving saidfirst secondary wheel, a second axleshaft driving said second secondarywheel, and a drive mechanism coupling said input shaft to said first andsecond axleshafts, said drive mechanism including a differential, aspeed changing unit, and first and second mode clutches, saiddifferential having an input component driven by said input shaft, afirst output component driving said first axleshaft and a second outputcomponent driving said second axleshaft, said speed changing unit havinga first shaft driven by said first output component, a second shaftoperably coupled to said second axleshaft, and first and second gearsetsdriven by said first shaft, said first mode clutch is operable forselectively coupling said first gearset to said second shaft fordecreasing the rotary speed of said second axleshaft, and said secondmode clutch is operable for selectively coupling said second gearset tosaid second shaft for increasing the rotary speed of said secondaxleshaft; and a control system for controlling actuation of said firstand second mode clutches.
 2. The motor vehicle of claim 1 wherein saiddrive mechanism is operable to establish a first drive mode when saidfirst mode clutch is engaged and said second mode clutch is released,whereby said second axleshaft is underdriven relative to said inputcomponent and said differential causes said first axleshaft to beoverdriven relative to said input component.
 3. The motor vehicle ofclaim 2 wherein said drive mechanism is operable to establish a seconddrive mode when said first mode clutch is released and said second modeclutch is engaged, whereby said second axleshaft is overdriven relativeto said input component and said differential causes said firstaxleshaft to be underdriven relative to said input component.
 4. Themotor vehicle of claim 1 wherein said drive mechanism establishes alocked mode when both of said first and second mode clutches areengaged.
 5. The motor vehicle of claim 1 wherein said differentialincludes a ring gear as its input component, a differential carrier asits first output component, a sun gear as its second output component,and planet gears supported by said differential carrier and which aremeshed with said ring gear and said sun gear.
 6. The motor vehicle ofclaim 5 wherein said first gearset includes a first drive gear driven bysaid first shaft that is meshed with a first speed gear rotatablysupported on said second shaft, and wherein said second gearset includesa second drive gear driven by said first shaft that is meshed with asecond speed gear rotatably supported on said second shaft.
 7. The motorvehicle of claim 6 wherein said first mode clutch includes a firstclutch pack disposed between said second shaft and said first speed gearand a first power-operated clutch actuator operable to generate andexert a clutch engagement force on said first clutch pack, wherein saidsecond mode clutch includes a second clutch pack disposed between saidsecond shaft and said second speed gear and a second power-operatedclutch actuator operable to generate and exert a clutch engagement forceon said second clutch pack, and wherein said control system includes acontrol unit operable to control actuation of said first and secondclutch actuators.
 8. A motor vehicle, comprising: a powertrain operablefor generating drive torque; a primary driveline for transmitting drivetorque from said powertrain to first and second primary wheels; asecondary driveline for selectively transmitting drive torque from saidpowertrain to first and second secondary wheels, said secondarydriveline including an input shaft driven by said powertrain, a firstaxleshaft driving said first secondary wheel, a second axleshaft drivingsaid second secondary wheel, and a drive mechanism coupling said inputshaft to said first and second axleshafts, said drive mechanismincluding a differential, a speed changing unit, and first and secondmode clutches, said differential having an input component driven bysaid input shaft, a first output component driving said first axleshaftand a second output component driving said second axleshaft, said speedchanging unit having a first shaft driven by said input component, asecond shaft operably coupled to said second axleshaft, and first andsecond gearsets driven by said first shaft, said first mode clutch isoperable for selectively coupling said first gearset to said secondshaft for decreasing the rotary speed of said second axleshaft, and saidsecond mode clutch is operable for selectively coupling said secondgearset to said second shaft for increasing the rotary speed of saidsecond axleshaft; and a control system for controlling actuation of saidfirst and second mode clutches.
 9. The motor vehicle of claim 8 whereinsaid drive mechanism is operable to establish a first drive mode whensaid first mode clutch is engaged and said second mode clutch isreleased, whereby said second axleshaft is underdriven relative to saidinput component and said differential causes said first axleshaft to beoverdriven relative to said input component.
 10. The motor vehicle ofclaim 9 wherein said drive mechanism is operable to establish a seconddrive mode when said first mode clutch is released and said second modeclutch is engaged, whereby said second axleshaft is overdriven relativeto said input component and said differential causes said firstaxleshaft to be underdriven relative to said input component.
 11. Themotor vehicle of claim 8 wherein said drive mechanism establishes alocked mode when both of said first and second mode clutches areengaged.
 12. The motor vehicle of claim 8 wherein said differentialincludes a drive carrier as its input component, first and second sidegears as its first and second output components, and pinion gears drivenby said carrier and meshed with said first and second side gears. 13.The motor vehicle of claim 12 wherein said first gearset includes afirst drive gear driven by said first shaft that is meshed with a firstspeed gear rotatably supported on said second shaft, and wherein saidsecond gearset includes a second drive gear driven by said first shaftthat is meshed with a second speed gear rotatably supported on saidsecond shaft.
 14. The motor vehicle of claim 13 wherein said first modeclutch includes a first clutch pack disposed between said second shaftand said first speed gear and a first power-operated clutch actuatoroperable to generate and exert a clutch engagement force on said firstclutch pack, wherein said second mode clutch includes a second clutchpack disposed between said second shaft and said second speed gear and asecond power-operated clutch actuator operable to generate and exert aclutch engagement force on said second clutch pack, and wherein saidcontrol system includes a control unit operable to control actuation ofsaid first and second clutch actuators.
 15. A drive axle assembly foruse in a motor vehicle having a powertrain and first and second wheels,comprising: an input shaft driven by the powertrain; a first axleshaftdriving the first wheel; a second axleshaft driving the second wheel; adrive mechanism coupling said input shaft to said first and secondaxleshafts, said drive mechanism including a differential, a speedchanging unit, and first and second mode clutches, said differentialhaving an input component driven by said input shaft, a first outputcomponent driving said first axleshaft and a second output componentdriving said second axleshaft, said speed changing unit having a firstshaft driven by said first output component, a second shaft operablydriven by said second output component, and first and second gearsetsdriven by said first shaft, said first mode clutch is operable forselectively coupling said first gearset to said second shaft, and saidsecond mode clutch is operable for selectively coupling said secondgearset to said second shaft; and a control system for controllingactuation of said first and second mode clutches.
 16. The drive axle ofclaim 15 wherein said drive mechanism is operable to establish a firstdrive mode when said first mode clutch is engaged and said second modeclutch is released, whereby said first gearset causes said secondaxleshaft to be underdriven relative to said first shaft and saiddifferential causes said first axleshaft to be overdriven relative tosecond axleshaft.
 17. The drive axle of claim 16 wherein said drivemechanism is operable to establish a second drive mode when said firstmode clutch is released and said second mode clutch is engaged such thatsaid second gearset causes said second axleshaft to be overdrivenrelative to said first shaft and said differential causes said firstaxleshaft to be underdriven relative to said second axleshaft.
 18. Thedrive axle of claim 17 wherein said drive mechanism establishes a lockedmode when both of said first and second mode clutches are engaged. 19.The drive axle of claim 15 wherein said differential includes a ringgear as its input component, a differential carrier as its first outputcomponent, a sun gear as its second output component, and planet gearsrotatably supported by said differential carrier and which are meshedwith said ring gear and said sun gear.
 20. The drive axle of claim 19wherein said first gearset includes a first drive gear driven by saidfirst shaft that is meshed with a first speed gear rotatably supportedon said second shaft, and wherein said second gearset includes a seconddrive gear driven by said first shaft that is meshed with a second speedgear rotatably supported on said second shaft.
 21. The drive axle ofclaim 20 wherein said first mode clutch includes a first clutch packdisposed between said second shaft and said first speed gear, and afirst power-operated clutch actuator operable to generate and exert aclutch engagement force on said first clutch pack, wherein said secondmode clutch includes a second clutch pack disposed between said secondshaft and said second speed gear, and a second power-operated clutchactuator operable to generate and exert a clutch engagement force onsaid second clutch pack, and wherein said control system includes acontrol unit operable to control actuation of said first and secondclutch actuators.
 22. A drive axle assembly for use in a motor vehiclehaving a powertrain and first and second wheels, comprising: an inputshaft driven by the powertrain; a first axleshaft driving the firstwheel; a second axleshaft driving the second wheel; a differentialhaving a ring gear driven by said input shaft, a carrier fixed forrotation with said first axleshaft, a sun gear fixed for rotation withsaid second axleshaft, and meshed pairs of first and second planet gearsrotatably supported by said carrier, said first planet gears are meshedwith said sun gear and said second planet gears are meshed with saidring gear; a speed changing unit having a first transfer shaft driven bysaid carrier, a second transfer shaft driven by said second axleshaft,an underdrive gearset driven by said first transfer shaft, and anoverdrive gearset driven by said first transfer shaft; a first modeclutch for selectively coupling said underdrive gearset to said secondtransfer shaft; a second mode clutch for selectively coupling saidoverdrive gearset to said second transfer shaft; and a control systemfor controlling actuation of said first and second mode clutches.
 23. Atransfer case for a four-wheel drive vehicle having a powertrain andfirst and second drivelines, comprising: an input shaft driven by thepowertrain; a first output shaft driving the first driveline; a secondoutput shaft driving the second driveline; a torque distributing drivemechanism operably interconnecting said input shaft to said first andsecond output shafts, said torque distributing drive mechanism includinga differential, a speed changing unit, and first and second modeclutches, said differential having an input component driven by saidinput shaft, a first output component driving said first output shaftand a second output component driving said second output shaft, saidspeed changing unit having a first transfer shaft driven by said firstoutput component, a second transfer shaft operably coupled to saidsecond output shaft, a first gearset having a first drive gear driven bysaid first transfer shaft and a first speed gear driven at a reducedspeed relative to said first transfer shaft, and a second gearset havinga second drive gear driven by said first transfer shaft and a secondspeed gear driven at an increased speed relative to said first transfershaft, said first mode clutch is operable for selectively coupling saidfirst speed gear to said second transfer shaft, and said second modeclutch is operable for selectively coupling said second speed gear tosaid second transfer shaft; and a control system for controllingactuation of said first and second mode clutches.
 24. The transfer caseof claim 23 wherein said drive mechanism is operable to establish afirst drive mode when said first mode clutch is engaged and said secondmode clutch is released such that said second output shaft isunderdriven relative to said input component and said first output shaftis overdriven relative to said input component.
 25. The transfer case ofclaim 24 wherein said drive mechanism is operable to establish a seconddrive mode when said first mode clutch is released and said second modeclutch is engaged such that said second output shaft is overdrivenrelative to said input component and said first axleshaft is underdrivenrelative to said input component.
 26. The transfer case of claim 23wherein said drive mechanism establishes a locked mode when both of saidfirst and second mode clutches are engaged.
 27. The transfer case ofclaim 23 wherein said first mode clutch includes a first clutch packdisposed between said second transfer shaft and said first speed gear,and a first power-operated clutch actuator operable to generate andexert a clutch engagement force on said first clutch pack, wherein saidsecond mode clutch includes a second clutch pack disposed between saidsecond transfer shaft and said second speed gear, and a secondpower-operated clutch actuator operable to generate and exert a clutchengagement force on said second clutch pack, and wherein said controlsystem includes a control unit operable to control actuation of saidfirst and second clutch actuators.